The use of optical fibers in communications is growing at an unprecedented rate. Low loss optical fibers which are produced by any one of several techniques may be assembled into ribbons which are then assembled into cables, or stranded into cables, or they may be enclosed singularly in a jacket and used in various ways in a central office, for example.
In order to assure that the low loss optical fibers which are produced today are not diminished in their effectiveness in systems, the fibers are connected through intermateable connectors which preserve those low losses. For optical fiber ribbons, connectors comprise grooved chips which hold a plurality of fibers of one ribbon in alignment with fibers of another ribbon. Such a connector is shown for example in U.S. Pat. No. 3,864,018 which issued on Feb. 4, 1975 in the name of C. M. Miller.
For single fiber cables, connections may be made through a connector which is referred to as a biconic connector. See, for example, an article entitled "Interconnection for Lightguide Fibers" which was authored by T. L. Williford et al. and which appeared in the Winter 1980 issue of the Western Electric Engineer beginning at page 87.
The biconic connector includes a coupling having a housing in which is mounted a biconic alignment sleeve. The sleeve includes two conically shaped cavities which communicate with each other through a common plane which has the least diameter of each cavity. Each of two optical fibers to be connected is terminated with a plug comprising a truncated, conically shaped end portion which is adapted to be received in one of the cavities of the sleeve. Each fiber extends through the plug in which it is mounted and has an end which terminates in an end face of the plug. The plug is held in a cap having an externally threaded portion adapted to be turned into an internally threaded entrance portion of the housing. Portions of the conically shaped surfaces of the plug and of the sleeve are conformable and serve as alignment surfaces. Each plug is urged into seated engagement with the wall defining the cavity in which it is received while its associated cap is turned into the housing. Minimal loss between the connected fibers is achieved when the optical fibers which are terminated by the plugs are aligned coaxially and when the fiber end faces, each of which is planar, contact in the common plane.
In at least one optical fiber cable of the prior art which is to be terminated with a plug, a core comprising at least one optical fiber is enclosed in a jacket, non-metallic filamentary strength members such as polymeric yarn, for example, and an outer jacket. It becomes very important to prevent the transmittal of tensile forces to optical fibers at their terminations with the plugs. If this is not done, the fibers may be broken or microbending losses in the fiber may occur and degrade the quality of the transmission. Also, any forces which are not diverted to other portions of the connector may be imparted to a plug thereby disturbing its seating in the sleeve and its critical alignment with the other plug.
In order to avoid such losses at a connection, provisions must be made for avoiding the application of forces to the optical fibers after portions of a sheath system of the cable have been removed for termination. Instead, any pulling forces must be transferred to the connector housing. When a connection is to be made, the strength members must be coupled to a housing portion of the connector so that forces are transferred to the housing before the forces reach the optical fiber terminations.
This requirement becomes even more important in special environments. For example, a new optical fiber assembly has been developed to withstand stringent environmental and mechanical requirements which are imposed on tactical as well as on commercial communications equipment. One requirement is that there is no attenuation increase at an operating tensile load of about 1700 newtons.
The transfer of forces from the cable to the connector housing instead of to the plugs must be made simple and through a termination of the strength members. In one prior art device, polymeric yarn is held between cooperating surfaces of internal and external sleeves. The internal sleeve includes a bore through which the optical fibers extend and the external sleeve includes a conically shaped bore for receiving the internal sleeve. The polymeric yarn strands, for example, are passed through the bore of the internal sleeve and retroflexed about a peripheral end portion of the internal sleeve whereafter the external sleeve is caused to be disposed over the internal sleeve. This arrangement could lead to abrasion of the polymeric yarn where it engages the peripheral end portion of the internal sleeve. Also, the application of tensile forces to the cable may cause dislocation and an undoing of the terminations of the strength members.
It also is important that there be minimum contact of each plug with supporting surfaces when the plug is engaged with a plug of another connector within an alignment sleeve. In this way, misalignment of the plugs in the sleeve is minimized. Further, it becomes important to prevent the ingress of moisture into the connector while allowing for longitudinal and radial movement of the plugs so that they may become suitably disposed in alignment sleeves.
Seemingly, the prior art is devoid of a connector in which non-metallic filamentary strength members are secured to a housing of the connector and maintained as such during the application of forces to the cable while not compromising the integrity of the strength members. Desirably, connection of the strength members to the connector is accomplished in a manner which causes the secured terminations to be enhanced when the cable is subjected to tensile forces. Also, the sought-after connector should be one which facilitates alignment of plugs of connectors in sleeves and which prevents the ingress of moisture.